The present invention relates to a liquid crystal mirror and, more particularly, to a mesopic-free liquid crystal mirror.
Vehicles generally include an interior rearview mirror and two side exterior rearview mirrors. The rearview mirrors allow the driver to view scenes behind the vehicle without having to face in a rearward direction and to view areas around the vehicle that would otherwise be blocked by the vehicle structure. As such, rearview mirrors are an important source of information to the driver. Bright lights appearing in a scene behind the vehicle, such as from another vehicle approaching from the rear, may create glare in a rearview mirror that can temporarily visually impair or dazzle the operator. This problem is only aggravated under low ambient light conditions such as at night, when the eyes of the driver have adjusted to the darkness.
Various solutions have evolved to deal with the problem of glare in rearview mirrors of vehicles. One conventional solution to this problem, used primarily with interior, center-mounted rear view mirrors, is to employ a prismatic mirror with a switch lever on the mirror housing.
The switch can be manually moved between a daytime position, providing direct, normal intensity reflection from the mirror surface, and a night time position providing a reduced intensity reflection. When the driver experiences glare, he manually changes the rearview mirror setting to low reflectivity. With the low intensity of light reflected to the driver, the intensity of reflected headlights from trailing vehicles is insufficient to impair the driver's vision. Once the glare is subsided the driver can manually switch the rearview mirror back to high reflectivity. Difficulties with manually controlled mirrors include the glare experienced before the mirror could be switched as well as driver distraction caused by finding and operating the switch lever.
Another solution is the use of mirrors which absorb yellow light and reflect light in all other visible colors, i.e., from red to orange and from green to purple (to this end see, e.g., U.S. Pat. Nos. 5,075,674 and 5,844,721). The motivation for manufacturing this type of mirror is the belief that yellow light is the source of most visual discomfort to the driver. The exclusion of yellow light is typically achieved using Neodymium Oxide, a rare earth oxide, either as a separate film within the mirror or as a dopant applied to the glass of the mirror.
However, at night, when the rods in the human visual system are dominant, there is a high sensitivity of the human visual system to the blue and near blue light. Therefore, although exclusive absorption of yellow light reduces glare discomfort, such technique is still far from being satisfactory because there is a considerable contribution of other colors of the spectrum to glare.
Also known in the art are automatically dimming rearview mirrors which eliminate the need for the operator to manually switch the mirror.
Early designs of such automatically dimming mirrors included a single glare sensor facing rearward to detect the level of light striking the mirror. This design, however, has been proved to be inadequate since the threshold perceived by the driver for dimming the mirror, known as the glare threshold, varies as a function of the ambient light level. An improved design is a dual sensor automatically dimming mirror which includes a second light sensor for detecting the ambient light level. The glare threshold in these systems is based on the amount of ambient light detected. Another related approach includes an imaging array which gathers light from behind and beside the vehicle. Ambient light is detected by examining pixels generally looking sideways. The cost of such a systems is, however, prohibitively expensive for many automotive applications.
Improvements in glare reduction additionally occurred when prismatic mirrors having two states were replaced with multi-state mirrors which include dimming elements capable of providing many levels of reflectivity reduction. One type of such multi-state automatically dimming rearview mirrors is based on the well known Stark effect, named after the German physicist and Nobelist Johannes Stark (1874-1957). According to the Stark effect, there is a splitting or shift of the spectral lines of atoms when present in an external electric field. The Stark effect is also referred to in the literature as the electrochromic effect and the automatically dimming rearview mirrors are commonly termed electrochromic mirrors.
An electrochromic mirror includes an electrochromic medium connected between two electrodes. Under the principles of the Stark effect, the electrochromic medium is responsive to external electric field generated by electrodes. When a sufficient electrical potential difference is applied across the electrodes of the automatically dimming rearview mirror the electrochromic medium enters a translucent state by changing its spectral characteristics. Typical electrochromic mirrors are described in many U.S. patents (to this end see, U.S. Pat. Nos. 4,902,108, 5,724,187, 5,679,283, 5,725,809).
WO/26633 teaches an electrochromic mirror assembly, formed of transparent semi-conductive layers and a reflective conductive layer. When no voltage is applied, the mirror is spectrally unselective in photopic reflectivity. The mirror exhibits a spectrally selective transmission characteristic, established by the refractive indices and physical thicknesses of the semi-conductive and conductive layers. When a voltage is applied across the mirror, the mirror exhibits a spectrally selective transmission characteristic. A display, controllable by application of voltage, is positioned within the assembly such that when a voltage is applied, the display emits light which is transmitted through the layers, being viewable by the driver.
Prior art electrochromic mirrors suffer from many limitations such as slow response rate, and high cost.
Other known automatically dimming mirrors make use of the properties of liquid crystals which are normally transparent to light but which when subjected to an electric field beyond a certain threshold, present a state of molecular realignment which is visibly different from the normal transparent state. While being in the molecular realignment state the light reflected from the mirrors is attenuated to a degree which is proportional to the applied electric field. Upon suppressing the applied electric field, the liquid crystal returns to the normal transparent state. Using such mirrors, therefore, it is possible to obtain selectively a high or a low reflecting power, according to whether the electrical voltage applied to the liquid crystal is lower or greater than the threshold.
Typical automatically dimming liquid crystal mirrors are found, e.g., in U.S. Pat. Nos. 4,660,937, 4,589,735 and 4,200,361. These and other prior art liquid crystal mirrors are costly, technologically difficult to employ, or otherwise suffer from poor performances.
The present invention provides solutions to the problems associated with prior art automatic dimming techniques.